Shader output as factor input for other shader nodes
Answer: Wasn't working as you can't plug BSDF into Color (rookie error).  I have fixed the problem (getting reflection and nice clear transmission by mixing a Transparency shader with Principled shader.  The Principled shader provides the reflection (which Transparency does not) and the Transparency shader gives a nice clear view through the glass without the thousands of passes needed for clear transmission.  Only missing the distortion that comes from refraction but an OK compromise for my purposes.